Buildings and other structures located in areas containing loose granular soils may be subject to excessive settlement as soil densifies and settles during static or dynamic loading. Soil densification by dynamic loads may be caused by reciprocating machinery, applications of dynamic loads such as wind loads, or by earthquakes. Earthquakes occur as a result of tectonic activity. When earthquakes occur, they shake the bedrock in the vicinity of the fault rupture that results in compressive and shearing stresses applied to the soil column above the rock.
Seismically-induced waves propagate upwards through the soil profile, often resulting in damage to existing structures. This damage can sometimes be caused by soil liquefaction that results from shaking. Liquefaction is a phenomenon that occurs in saturated soils that involves the transfer of the effective overburden load from the soil grains to the pore fluid, with the commensurate reduction in effective stress and, hence, reduction in soil strength. Pore fluid is the groundwater held within a soil or rock; namely, in the gaps between particles (i.e., in the pores). Pore water pressure refers to the groundwater pressure within the pores of the soil or rock. In earthquake-induced liquefaction, this transfer is initiated in sandy soils by the collapse of the soil skeleton due to earthquake shaking. Following liquefaction, settlement occurs as the pore water pressures dissipate. Soil liquefaction can result in billions of dollars in structural damage and can lead to a loss of life. Examples of the devastating effects of soil liquefaction can be found in the aftermath of destruction from the recent Haiti, Conception Chile, and Christchurch New Zealand earthquakes.
One way to support structures to minimize damage from the densifying of loose soil during static and dynamic loading is by using deep foundation elements. Such deep foundations are typically made from driven pilings or concrete piers installed by drilling. The deep foundations are designed to transfer structural loads through the soft and loose soils to more competent soil strata. Deep foundations are often relatively expensive when compared to other construction methods. Further, the design of deep foundation elements must consider the deleterious effects of liquefaction such as reduction in the supporting capacity of the now liquefied soil in response to applied vertical and lateral loads.
More recently, ground reinforcement with aggregate columns has been used to support structures located in areas containing loose and weak soil. The columns are designed to reinforce and strengthen the soft layers and reduce settlements. Such piers are constructed using a variety of methods. For example, piers that are constructed using drilling and tamping methods are described in U.S. Pat. No. 5,249,892, entitled “Short Aggregate Piers and Method and Apparatus for Producing Same,” issued on Oct. 5, 1993; and U.S. Pat. No. 6,354,766, entitled “Methods for Forming a Short Aggregate Pier and a Product Formed from said Methods,” issued on Mar. 12, 2002. Piers that are constructed using driven mandrel methods are described in U.S. Pat. No. 6,425,713, entitled “Lateral Displacement Pier, and Apparatus and Method of Forming the Same,” issued on Jul. 30, 2002. Piers that are constructed using tamping head driven mandrel methods are described in U.S. Pat. No. 7,226,246, entitled “Apparatus and Method for Building Support Piers from One or Successive Lifts Formed in a Soil Matrix,” issued on Jun. 5, 2007; and U.S. Pat. No. 7,326,004, entitled “Apparatus for Providing a Rammed Aggregate Pier,” issued on Feb. 5, 2008. Each of these methods requires that aggregate, such as crushed limestone, be imported to the site and placed in the cavity and is generally only efficient to depths of 40 feet (12.2 m).
As an alternative to deep foundations and aggregate columns, the loose sand can be excavated and then the excavation refilled with more competent material. This method is advantageous because it is performed with conventional earthwork methods, but has the disadvantages of (1) being costly when performed in urban areas; (2) may require costly dewatering or shoring be performed to stabilize the excavation; and (3) is often impractical and environmentally insensitive.
Alternatively, the loose sand can be densified in-place. One way to perform soil densification in-place is by using a technique known as “deep dynamic compaction.” Deep dynamic compaction consists of dropping a heavy weight on the ground surface in order to cause a large compression wave to develop in the soil, wherein the compression wave compacts the soil (provided the soil is of a sufficient gradation to be treatable). A variety of weight shapes are available to achieve compaction by this method, such as those described in U.S. Pat. No. 6,505,998, entitled “Ground Treatment,” issued on Jan. 14, 2003. While deep dynamic compaction may be economical for certain sites, it has the disadvantage that it induces large waves in the soil. These waves may be damaging to surrounding structures. The technique is also deficient because it is only applicable to a small band of soil gradations (particle sizes) and is not suitable for materials with appreciable fine-sized particles. Deep dynamic compaction is further limited by practical treatment depths of 30 ft (9.1 m) or less.
Yet another way to perform soil densification is by using a technique known as vibroflotation, wherein vibrators are lowered into the ground. While vibroflotation methods are effective at treating liquefaction, vibroflotation methods may be slow and require powerful mechanical vibrators that consume large amounts of energy.
Still another way to perform soil densification is by explosive methods (i.e., explosive blasting using TNT or other chemical explosives placed within boreholes). Explosive blasting causes shock waves to be generated in the ground after the explosive charges have been detonated. While explosive blasting has been used successfully at great depths below dams and other large structures, blasting is dangerous and requires great care in its execution.
Yet another recent method of providing soil densification includes the “Densipact®” method, such as described in U.S. Pat. No. 8,328,470, entitled “Apparatus and Method for Ground Improvement,” issued on Dec. 11, 2012. In the '470 patent, a tool utilizing a plurality of downwardly extending tines is driven into the soil in order to displace the ground material downward and radially outward. Repeated retraction and driving of the tines can achieve densification.
The present invention is an improvement on such prior techniques, and in particular, deep dynamic compaction and vibroflotation. Deep dynamic compaction and vibroflotation both decrease static and dynamic settlement potential by densifying deposits of clean granular soils. Deep dynamic compaction is generally only efficient at improving the relative density of soil deposits less than 30 ft (9.1 m) in depth. Vibroflotation requires the operation of a powerful mechanical vibrator, a process that consumes energy and is relatively slow. The present invention is not limited by depth and can be performed with relatively small equipment and relatively quickly.